The invention relates to a logic input circuit for an industrial equipment automatic control system supplied by a DC voltage source, in particular a battery, said input circuit comprising means for detection designed to deliver a measurement signal representative of the logic state when opening and closing of an input contact take place, with a very low heat dissipation.
A logic input board in electronic equipment for automatic control systems of in-board installations, in particular of rail transport vehicles, generally requires a large input current of about 10 mA. The problem arising in conventional equipment is then that of heat dissipation in the measuring resistors, as a logic input in a voltage of 110 Volts for example, and a current with an intensity of 10 mA, will dissipate a thermal power of 1.1 W. The global heat dissipation is naturally proportional to the number of logic inputs and often requires the use of cooling devices by heat sink or heat pipes to remove the heat to the outside. As electrical power supply of the equipment is performed from a safety power system with a battery bank, this results in addition in a loss of electrical power of the vehicle.
The object of the invention is to achieve a logic input circuit for an automatic control system, with voltage matching over a wide range of input voltages, and presenting a very low heat dissipation.
The input circuit according to the invention is characterized in that it comprises:
a voltage step-up energy converter composed of an inductance coil and a switching transistor, connected to the input of the circuit,
a logic level detector connected either between the inductance coil and a rectifier diode of an energy recovery circuit or between a charging resistor and the DC voltage source,
and a clock circuit controlling switching of the transistor by adjustment of the frequency and duty cycle to perform voltage matching with the signals applied to the input, and also the value of the voltage surge generated in logic high state by the inductance coil when switching of the transistor to the off state takes place.
Detection of the logic state is performed with a very low heat dissipation avoiding the use of heat sinks. Most of the energy is restored to the battery by the energy recovery circuit. The presence of the energy converter associated to the inductance coil avoids voltage step-up transformers having to be used.
The logic level detector advantageously comprises an optocoupler having an emitting diode through which the measuring current flows, and a receiver delivering an image of the measurement signal to an acquisition circuit.
According to one feature of the invention, the energy recovery circuit is provided with a charging resistor connected in series between the rectifier diode and the battery, or the emitting diode of the logic level detector. A first filtering capacitor is connected in parallel between the input potential reference and the mid-point of connection of the rectifier diode to the resistor. A second filtering capacitor is connected between the two input terminals.